Method of improving the corrosion resistance of nickel-copper alloys

ABSTRACT

The corrosion resistance of nickel-copper alloys containing 6370 wt. percent nickel to water is improved by immersing the alloy in concentrated hydrochloric acid for 24 hours at room temperature prior to exposure of the alloy to water.

United States Patent 1 Tokarz 51 May 13, 1975 METHOD OF IMPROVING THECORROSION RESISTANCE OF NICKEL-COPPER ALLOYS [75] Inventor: Richard D.Tokarz, Ruhland, Wash.

[73] Assignee: The United States of America as represented by the UnitedStates Energy Research and Development Administration, Washington, DC.

221 Filed: Dec. 21, 1972 211 Appl. No.: 317,456

[52] US. Cl. 156/18; 252/792; 252/387 [51] Int. Cl C231 l/00; C23f 15/00[58] Field of Search 156/2, 3, 7, 18; 252/792,

[56] References Cited UNITED STATES PATENTS 2.680.678 6/1954 Fox 156/203,702,273 11/1972 Johnston et al 156/18 OTHER PUBLICATIONS PrimaryExaminerCharles E. Van Horn Attorney, Agent, or FirmDean E. Carlson;Arthur A. Churm; Frank H. Jackson [57] ABSTRACT The corrosion resistanceof nickel-copper alloys containing 63-70 wt. percent nickel to water isimproved by immersing the alloy in concentrated hydrochloric acid for 24hours at room temperature prior to exposure of the alloy to water.

2 Claims, No Dravlrings METHOD OF IMPROVING THE CORROSION RESISTANCE OFNICKEL-COPPER ALLOYS CONTRACTUAL ORIGIN OF THE INVENTION The inventiondescribed herein was made in the course of, or under, a contract withthe UNITED STATES ATOMIC ENERGY COMMISSION.

BACKGROUND OF THE INVENTION This invention relates to a method ofimproving the corrosion resistance of nickel-copper alloys to water. Inmore detail, the invention relates to a method of increasing thecorrosion resistance of 63 to 70 wt. percent nickel-copper alloys to hotwater.

High nickel-copper alloys containing 63 to 70 wt. percent nickel areextensively used where good corrosion and heat resistance are required.Varieties containing minor amounts of other constituents have otherdesirable properties. Unfortunately, experience has shown that bodiesformed of these alloys begin pitting shortly after exposure to water ata high temperature about 170 C and extensive failures are known to haveoccurred after 2 years service.

SUMMARY OF THE INVENTION According to the present invention, highnickelcopper alloys such as Monel alloy R-405 which contains 63-70 wt.percent nickel and a maximum of 0.30 wt. percent carbon, 2.0 wt. percentmanganese, 2.5 wt. percent iron, 0.025-0.060 wt. percent sulfur and 0.50wt. percent silicon, the balance being copper is immersed inconcentrated hydrochloric acid at room temperature for about 24 hours toetch the surface thus increasing the resistance of the alloys to water.The invention is particularly useful when the alloy is expected to be incontact with hot water for a long period of time.

SPECIFIC EMBODIMENT OF THE INVENTION Immersion-type resistancetemperature detectors enclosed in Monel alloy R-405 have been used inreactor service at Hanford Engineer Works, Richland, Washington. Thetemperature of the water in which the temperature detectors wereimmersed ranged from 20 to 170C.

According to a 1969 bulletin of the Huntington Alloy Products Divisionof The International Nickel Company, Inc., on Monel nickel-copperalloys, alloy R-405 (formerly known as R Monel) contains 63.0 to 70.0wt. percent nickel (plus cobalt) and a maximum of 0.30 wt. percentcarbon, 2.0 wt. percent manganese, 2.5 wt. percent iron, 0.025 to 0.060wt. percent sulfur, and 0.50 wt. percent silicon, the balance beingcopper. The nominal composition of one specific Monel R-405 alloy is66.7 wt. percent nickel, 30 wt. percent copper, 1.7 wt. percent iron,1.1 wt. percent manganese, 0.1 wt. percent carbon, 0.05 wt. percentsilicon and 0.035 wt. percent sulfur. In addition, I have determinedthat Monel contains a small quantity of magnesium present in the form ofstringers. Approximately 90 percent of the detectors installed on oneparticular reactor began pitting shortly after installation. Failuresdue to pinholes in the Monel metal after two years service causedextensive effort to stop the pitting action on the alloy.

Metallurgical studies indicated that both Monel alloy R-405 which wasthe alloy used and Monel alloy 400 contain magnesium sulfide stringers,Monel alloy R-405 having more stringers intentionally introduced toimprove machinability. Monel alloy 400 has the same composition as alloyR-405 except that the amount of sulfur present is lower. Since magnesiumis employed as a deoxidizer when forming the alloy, it is believed thatthe magnesium in the stringers is residual from the forming process. Theamount is very small, probably less than 0.5 wt. percent. It is believedhighly probable that the magnesium is present in the stringers in theform of magnesium sulfide. Our belief is based on the fact that chemicalanalysis of the stringers established the presence of magnesium andsulfur and magnesium was found to be present in every sample of Moneltested. Since the stringers dissolve instantly in tap water and the onlycompound of magnesium and sulfur which is soluble in water is magnesiumsulfide, the stringers must be composed of magnesium sulfide.

Immersing the Monel alloy R-405 in nominally 38 percent hydrochloricacid (reagent grade concentrated HCI) at room temperature results in thefollowing. Magnesium sulfide stringers exposed to the surface aredissolved and removed from the metal. In addition, the difference indissolving rates of copper and nickel coupled with their relativeoxidation potentials causes the surface molecules of copper to go intosolution and leaves the metal surface relatively high in nickel content.Thus, the surface of the treated metal is relatively high in nickelcontent and contains no magnesium sulfide stringers exposed to thesurface.

A direct comparison has been made between resistance temperaturedetectors including a Monel alloy R-405 body which has been etchedaccording to the present invention and detectors including Monel alloyR-405 bodies that have not been so etched. Photographs clearly show thatdetectors which have been etched in hydrochloric acid show very littlepitting after 2 years in a reactor in a water environment, whilenonetched detectors under the same conditions pitted heavily in 3 or 4months.

It is, of course, evident that the hydrochloric acid concentration, timeand temperature may be varied within rather wide limits.

While this invention was developed for the purpose of improving thecorrosion resistance of Monel alloy exposed to high temperature waterand it is here that the invention is most important, the inventionshould increase the useful life of the alloy in any temperature water.

The embodiments of the invention in which an exclusive property orprivilege is claimed are defined as follows:

1. A method of improving the corrosion resistance to hot water of aresistance temperature detector including a body formed of anickel-copper alloy containing 63-70 wt. percent nickel and minoramounts of sulfur, said alloy further containing magnesium residual fromthe forming process wherein it had been used as a deoxidizer, comprisingetching the detector body in concentrated hydrochloric acid prior toexposure of the detector to hot water.

2. A method according to claim 1 wherein the alloy contains 63-70 wt.percent nickel (plus cobalt) and a maximum of 0.30 wt. percent carbon,2.0 wt. percent maganese, 2.5 wt. percent iron, 0.025 to 0.060 wt.percent sulfur and 0.50 wt. percent silicon, the balance being copperand wherein the detector body is etched in nominally 38 percenthydrochloric acid (reagent grade concentrated hydrochloric acid) for 24hours at room temperature.

1. A METHOD OF IMPROVING THE CORROSION RESISTANCE TO HOT WATER OF ARESISTANCE TEMPERATURE DETECTOR INCLUDING A BODY FORMED OF ANICKEL-COPPER ALLOY CONTAINING 63-70 WT, PERCENT NICKEL AND MINORAMOUNTS OF SULFUR, SAID ALLOY FURTHER CONTAINING MAGNESIUM RESIDUAL FROMTHE FORMING PROCESS WHEREIN IT HAD BEEN USED AS A DEOXIDIZER, COMPRISINGETCHING THE DETECTOR BODY IN CONCENTRATED HYDROCHLORIC ACID PRIOR TOEXPOSURE OF THE DETECTOR TO HOT WATER.
 2. A method according to claim 1wherein the alloy contains 63-70 wt. percent nickel (plus cobalt) and amaximum of 0.30 wt. percent carbon, 2.0 wt. percent maganese, 2.5 wt.percent iron, 0.025 to 0.060 wt. percent sulfur and 0.50 wt. percentsilicon, the balance being coppeR and wherein the detector body isetched in nominally 38 percent hydrochloric acid (reagent gradeconcentrated hydrochloric acid) for 24 hours at room temperature.